'Shake' defects and wood structure variations in British oaks (Quercus spp.)
The overall aim was to gain information required for improvement of wood quality in British oaks (Quercus robur and petraea). Specific aims were 1) to investigate the nature and incidence of shake in British oaks; 2) to quantify variations in oak wood structure and properties, identifying which characteristics are under genetic control and which associated with growth rate; 3) to make prescriptions for selection and silviculture of oak, with a view to improving wood quality in future British crops. Site surveys (including soil analyses) of 42 woodlands were carried out. An index of severity was devised to standardise shake assessment within individual trees. Wood structure was analysed in oaks from shake-prone and sound woodlands. Associations of wood structure and property variations with genotype and growth rate were analysed in oaks from a seed origin trial. Environmental factors were strongly associated with shake. High shake incidence occurred on sites which gave poor rooting conditions for oak: shallow, nutrient poor soils with low clay content and low calcium availability, and with soil texture and/or site topography leading to seasonal droughtiness or waterlogging. Woodland type (shake-prone or sound) was a stronger influence than tree condition (shaken or sound) on wood structure: oaks from shake-prone woodlands had wider rings, smaller earlywood percentage, larger wide rays and larger earlywood vessel radial diameters. Density, sapwood width, earlywood vessel frequency and proportions of wide rays and of latewood vessels and fibres varied between seed origins; these variables therefore have selection potential for improvement of oak wood quality. Many parameters also varied with growth rate, and earlywood vessel radial diameter was strongly associated with current ring width. A model of shake development is proposed in which various predispositions (structural weaknesses) and triggers (mechanical stresses) influence a tree over time. It is proposed that predispositions are caused by environmental factors at the time of wood formation, resulting in physiological stress or cambial damage. Genotype may modify response or susceptibility to environment. Predispositions may be extended by secondary weakening. Triggers of shake are thought to be natural internal growth stresses supplemented by the action of external forces (such as wind). Recommendations are made for site choice and silviculture of future British oak crops.